The present invention relates in general to CT scanning and more particularly concerns novel apparatus and techniques for reducing dosage to a patient without sacrifice in image quality.
The commercially available CT scanner from American Science and Engineering, Inc. comprises a circular array of 600 contiguous detectors illuminated in sequence by a rotating X-ray source that emits a fan beam. Those detectors within the arc of the fan beam at any instant of time during a scan are "active" and are periodically sampled to provide X-ray transmission data. The array of data associated with a given detector during a scan is called a "detector fan" or "data fan". It is important that each detector fan be obtained smoothly and continuously in time without significant interruption between adjacent samples or rays. Furthermore, it is important to have the data obtained at the beginning and at the end of the scan blend smoothly together. Compliance with these two criteria avoids discontinuities within individual fans and between spatially adjacent fans obtained at the beginning and at the end of the scan that would result in artifacts in the reconstructed CT image.
To achieve both smoothness and continuity in time, it is preferred to scan somewhat in excess of 360.degree. and to select suitable parts of the totality of data thus obtained as explained in greater detail below. A disadvantage of this approach is that some detectors are thus illuminated twice, providing some data which obeys the smoothness and continuity criteria, and additional data which is discarded because it violates the criteria. The unused data represents wasted dose to the patient.
It is an important object of the invention to provide an improved CT scanner.
It is another object of the invention to achieve the preceding object while reducing the dosage to which a patient is subject.
It is a further object of the invention to achieve one or more of the preceding objects without sacrifice in the quality of the reconstructed CT image.
It is still a further object of the invention to achieve one or more of the preceding objects with relatively little additional structure that is relatively inexpensive, reliable and relatively easy to incorporate into a CT scanner having a stationary array of detectors.